1. Field of Invention
This invention relates to systems and methods for optimally rotating high addressability images. More specifically, this invention relates to systems and methods for rotation of high addressability halftoned images using filtering, resampling and halftoning techniques while minimizing the presence of contours in the rotated image.
2. Description of Related Art
The digital reproduction, transfer or display of various images presently occurs using a variety of devices and systems in a variety of environments. The image may be input into a device, processed in some manner, and then output from the device, for example. In some applications, it may be necessary or desirable to convert the image between the input and the output of one device for the specific purpose of using the converted image data by another device. In other applications, it may be necessary or desirable to convert the input image for some particular application within a device itself.
Images are represented in a wide variety of manners using various techniques. Illustratively, an image may be represented in the form of a grayscale image commonly referred to as a continuous tone image. In such a representation, multiple grayscale values are used to create the varying portions of the image. Such a grayscale image may be composed of pixels that possess values in the range of 0-255, for example, resulting in the image possessing 256 possible grayscale values.
Further, images may be represented in binary form. Illustratively, a continuous tone image may be converted or xe2x80x9chalftonedxe2x80x9d and represented in a binary form. In a binary form, an image is represented by creating halftone cells or dots. Each cell represents a grayscale value within an area of pixels. The pixels in the binary image may be either on or off, i.e., black(1) or white(0), respectively. By turning the pixels in an area of the binary image on or off, a grayscale value may be simulated. As a result, the binary image can replicate the entire grayscale image without using continuous tones.
In particular, the binary image may be a high addressability binary image. A high addressability binary image is an image created by a device such that the spatial addressability of the writing spot is finer than the size of the writing spot. High addressability also often refers to an addressability resolution in a first direction is finer than the spatial addressability resolution in a second direction perpendicular to the first direction, for example.
Illustratively, FIG. 1 is a diagram showing a high addressability pixel grid. As shown in FIG. 1, the spatial addressability of the pixels in the horizontal direction, i.e., the fast scan direction, is finer than in the vertical direction, i.e., the slow scan or process direction. For a flying spot laser scanner, the fast scan direction is the direction in which a laser beam of a printer, for example, sweeps to print an image on a recording medium. The recording medium may be a xerographic photoreceptor that will develop and transfer onto a sheet of paper, for example. The photoreceptor is advanced in a direction perpendicular to the fast scan direction, i.e., the slow scan or process direction. The photoreceptor may be advanced using rollers for a belt-type device, or as a rotating drum, as is commonly used in a printer, for example. Note that other writing devices also have high addressability capability such as an LED image bar writer. In these other devices the orientation of the grids may be rotated, but the underlying concept is the same.
FIG. 1 shows the size of a nominal pixel and a high addressable pixel, as well as the size of a writing spot. The addressability in the fast scan direction is controlled by a laser beam modulator, for example. The addressability in the slow scan or process direction is controlled by the photoreceptor advance mechanism of the printer or copier. The laser beam is capable of modulation to a resolution of the high addressable pixel. However, the photoreceptor advance mechanism is not capable of such fine resolution. Rather, the paper feed mechanism is only capable of a nominal pixel resolution.
Various methods for image processing are known. These methods may encompass processing using scanning, or other image acquisition, in conjunction with printing or display of the image. Input scanners typically acquire image information possessing 256 levels of gray to represent a spot or pixel in the scanned image. In general, image output devices such as printers, for example, are only capable of creating spots within an area with a limited predetermined spatial resolution. In contrast to the gray-scale resolution of a scanner, output devices generally use only two gray-scale levels, or some other relatively small number of levels, available to reproduce image information. As a result, output devices commonly contend with excess gray-scale resolution information by quantizing the image data through halftoning techniques to represent the image as a halftone, i.e., a binary image possessing two grayscale levels.
Conventional digital halftoning devices can suffer from image quality degradations, such as too few perceived gray levels. One solution is to perform halftoning on a very high resolution device. Such a high resolution device may have resolution equal to or greater than 2400 spots per inch, for example. However, using high addressability techniques, a device may be able to achieve a sufficient number of perceived gray-levels without resorting to increasing the full spatial resolution in both the fast scan and process directions. Illustratively, high addressability methods conventionally typically involve modulating a writing member, such as a laser beam, at spatial increments finer than the size of the writing spot. Using high addressability imaging and modulation allows a particular device""s spatial resolution to be improved or increased.
Accordingly, high addressability techniques use modulation to increase printer spatial resolution without modifying the physical printer device. As described above, high addressability techniques may be used to affect the horizontal spatial resolution. For example, doubling the printer modulation rate results in doubling the horizontal spatial resolution, while keeping the vertical spatial resolution unchanged.
However, problems are present in conventional methods when inputting, manipulating and outputting high addressability binary images. In particular, problems occur in the conventional methods when rotating high addressability binary images. In a variety of devices and operating environments, it is often necessary or desirable to rotate images. The image may be a grayscale image or a binary image. More specifically, the binary image may be a high addressability image. In conventional methods and systems, rotating a grayscale image typically does not introduce defects or artifacts into the image. However, rotating a binary, high addressability image can result in gray-level contours in halftones and jagged edges in the line art and text portions of the rotated image.
Conventional rotation methods can cause defects and artifacts in an image. Further, conventional rotation methods are typically intended for use in rotating images that are of isomorphic resolution (same in vertical and horizontal directions). For example, a conventional technique may be used to rotate an image of pixels by xe2x88x9290xc2x0. As a result of this rotation with high addressability images, gray-level contours may be introduced into the rotated image. These contours, and other image artifacts, introduced using conventional rotation techniques, are highly objectionable.
Accordingly, this invention provides systems and methods that optimally rotate a high addressability binary image.
This invention separately provides systems and methods for rotating a high addressability binary image without increasing the bit count of the image.
This invention separately provides systems and methods for rotating high addressability binary images without introducing gray-level contours.
This invention separately provides systems and methods for rotating high addressability images that minimize any introduced graininess in a rotated image.
This invention separately provides systems and methods for rotating high addressability images that minimize any introduced moirxc3xa9 or pattern artifacts in a rotated image.
This invention separately provides systems and methods that slightly blur the high addressability binary image, convert the image into a grayscale image, convert the image into a isomorphic image using resampling, rotate the image, and convert the rotated image back to a high addressability binary image.
In one exemplary embodiment of the systems and methods of the invention, contour-free rotation of high addressability halftone images is obtained. The systems and methods of this invention low-pass filter and resample the high addressability binary image. The filtered and resampled image data results in a quantization of the high addressability binary image and forms a hybrid binary image, i.e., a quasi-grayscale image. The hybrid binary image is then resampled to isomorphic resolution and rotated. In particular, the hybrid binary image is rotated using conventional rotation techniques, and is rotatable without increasing the bit count and without introducing any gray-level contours. After rotation, the hybrid binary image may be processed using a high addressability halftoning technique preferably having some randomness, such as an error diffusion process, for example, to obtain a rotated high addressability image.
In accordance with another exemplary embodiment of the systems and methods according to the invention, an anamorphic high addressability binary image is input. The xe2x80x9canamorphicxe2x80x9d image, due to the high addressability, possesses different spatial resolutions in mutually perpendicular directions. For example, the spatial resolution in the horizontal direction is finer than the spatial resolution in the vertical direction. In accordance with the systems and methods of the invention, the anamorphic high addressability binary image is quantized to convert it to an isomorphic quasi-grayscale image. This isomorphic quasi-grayscale image possesses the same spatial resolution in mutually perpendicular directions. For example, the spatial resolution in the horizontal direction is the same as the spatial resolution in the vertical direction.
The quasi-grayscale image is not a true grayscale image. That is, a true grayscale image includes pixels that may possess any one of 256 grayscale values, in a range of 0-255. The quasi-grayscale image processed in accordance with the systems and methods of this invention is not so finely quantized. Rather, the quasi-grayscale image may perhaps possess only four levels of quantization for a system with resolution four times greater in one dimension than the other, for example.
Once the input image is quantized, the isomorphic quasi-grayscale image is rotated using conventional rotation techniques. After rotation, the rotated isomorphic quasi-grayscale image is processed using a halftoning technique, i.e., a high addressability halftoning technique, to generate a rotated high addressable anamorphic binary image. It should also be appreciated that the systems and methods according to the invention are applicable to a wide variety of image processes, beyond rotating a high addressability binary image prior to printing.
Thus, it should be recognized that the systems and methods described herein can be used in conjunction with various other processes and systems. For example, the systems and methods disclosed herein may be used in conjunction with or combined with the systems and methods disclosed in co-pending U.S. patent application Ser. No. 09/233,266 filed herewith, which is directed at optimally rotating line art, for example, and which is incorporated herein by reference in its entirety.
The systems and methods of the invention utilize a series of conventional image processing techniques in a series of steps. Further, the systems and methods of the invention do not require the increase of the bit count of an image to perform the rotation of the image. Since the bit count is not increased, the cost of implementing the rotation process in accordance with the invention is minimized.
These and other features and advantages of the systems and methods of this invention are described in or are apparent from the following detailed description of the exemplary embodiments.